Radio transceivers used in mobile communication require stable clock sources and frequency synthesizers for use in modulation and demodulation of RF signals. The frequency synthesizers must be capable of tuning and locking onto a base station frequency with high accuracy. In a typical cellular communications system, for example, a radio transceiver must typically be capable of initial tuning, based on dead reckoning to an accuracy of 2 ppm, and then once a base station signal is detected, locking onto the signal frequency to an accuracy of 0.2 ppm. In the 800 MHz cellular baud, these requirements translate into 1.6 kHz initial tuning accuracy and 160 Hz locking accuracy. In the 1.5 GHz band, the figures are roughly double.
Crystal oscillators, which are commonly used for frequency control in radio transceivers and other applications, are not in themselves sufficiently stable to provide the levels of accuracy noted above. The inherent frequency of the crystal oscillator is known to vary with temperature and also to change gradually as the crystal ages. A typical crystal oscillator, for example, a PXO-type oscillator, produced by Telequarz of Nelcarbischofsheim, Germany, has a frequency accuracy of ±10 ppm. The long-term drift of the frequency of the crystal with age is about 2 ppm per year.
Because of these shortcomings of ordinary crystal oscillators, transceivers known in the art generally use either a temperature-compensated crystal oscillator (TCXO) or a voltage-controlled TCXO (VCTCXO) as a precise frequency reference source. Temperature-compensated oscillators are described, for example, in U.S. Pat. Nos. 3,938,316, 4,015,208, 4,454,483, 5,375,146 and 5,604,468. The TCXO or VCTCXO receives an indication of the ambient temperature and corrects the oscillator output frequency accordingly, so as to compensate for the known characteristic temperature variation of the crystal frequency.
TCXOs and VCTCXOs are larger and heavier than simple crystal oscillators and include elements that are not easily incorporated in monolithic transceiver devices. They also depend on the use of a crystal which has a high motional capacitance to allow for externally-controlled tuning of the oscillator, and is therefore larger than standard crystals used in simple crystal oscillators. Therefore, TCXOs and VCTCXOs add undesirably to the cost and weight of RF communication devices, such as cellular telephones, in which they are used.